The present invention is directed to a test arrangement for non-contacting identification of defects in a non-structured surface. The test arrangement includes means for generating a scanning light ray, means for directing the light ray onto a non-structured surface, means for collecting light deflected at a defective location including an opto-electronic receiver for the detection of the collected light.
For locating defects in a non-structured surface, the surface of the unit under test, which is to be investigated, is scanned with a laser beam. The relative motion between the laser beam and the unit under test required for this purpose is thereby usually generated in the first direction by a scanning mirror arranged in the beam path and is generated in the second direction perpendicular to the first direction by displacing the unit under test. When the laser beam strikes a defective location, then the light is scattered or, respectively, defracted and can be detected as either reflected light or transmitted light via an opto-electronic receiver which supplies a signal corresponding to the detected defect. In order to supply the opto-electronic receiver with an optimumly great part of the light deflected at a defective location, this light is previously collected in a corresponding optical means. A hemisphere acting on the principle of Ulbricht's sphere is used, for example, as an optical collecting means and the hemisphere integrates the individual light rays coming from a defective location and thus, fully acquires them via the inside surface of the hemisphere. In addition to the spherical shape, however, many other shapes standard for optical collecting elements, such as elliptical hollow members, ellipsoids, parabolic hollow members of paraboloids and the like can be utilized.
In the known test arrangements, which are also frequently referred to as laser scanners, the laser beams employed as the scanning light ray has a relatively large halo, which in particular is attributed to light scatter of lenses, mirrored surfaces and the laser itself. This halo then causes unfavorable signal-to-noise ration so that smaller defects can no longer be reliably identified.
However, when checking as yet unstructured mask for semi-conductor technology, compact disk for either video or audio storage, and the magnetic storage disk, it is precisely a matter of acquiring even the smallest defect in the surface or in individual layers, and these defects may have a dimension in a sub-micrometer range.